The present invention is related to reentry vehicles that pass through the Earth's atmosphere on their final trajectory, and, in particular, relate to the materials that compose the radar window that is used in these reentry vehicles.
The development of materials characterized by an improved resistance to thermal ablation, with a high threshold for thermal and mechanical fracture has become a problem of particular importance with the recent advances achieved in rocket technology and especially in the development of antenna windows for advanced reentry vehicles. A number of materials have been suggested during the many attempts to solve these problems. For example, various ceramic oxides and nitrides such as aluminum oxide, beryllium oxide, magnesium oxide and boron nitride have been evaluated for use as antenna window material in advanced reentry vehicles because of their combined refractoriness and high temperature dielectric properties. However, the oxides are susceptible to failure by thermal fracture and rapid loss by melting at ultra high temperatures while boron nitride, which decomposes by sublimation at reentry temperatures, is subject to mechanical failure caused by shock wave loading.
The use of boron nitride fiber yarn, in particular, has allowed its incorporation into a more desirable composite form and, thus, has potential use as an antenna window. Ablation data has indicated that boron nitride three dimensionally reinforced composite has a recession rate similar to tape-wrapped carbon phenolic (TWCP) heat shield material used in reentry vehicles.